The reliability of telecommunication systems that users have come to expect and depend on is based, in part, on the systems' reliance on redundant equipment and power supplies. Telecommunication switching systems, for example, route tens of thousands of calls per second. The failure of such systems, due to, for instance, the loss of incoming AC power, may result in a loss of millions of telephone calls and a corresponding loss of revenue.
Power plants, such as battery plants, attempt to alleviate the power loss problem by providing the telecommunication system with a backup power supply for use in the event that the incoming source AC power is interrupted. Since the backup power supply is often called upon to provide power to the load for durations longer than just a few seconds, the implementation of a battery backup system has a significant impact on both the performance and the cost of the power plant.
Traditionally, the telecommunications systems are located in a central office environment wherein large-capacity power plants provide an energy reserve adequate to power the systems for as much as a few days. A power plant based on a DC-bus architecture, for instance, usually contains several rectifiers that process the incoming AC power and produce DC power that is then applied to a DC battery bus. The power plant also includes a plurality of batteries coupled to the DC battery bus, that provide an energy reserve in the event the incoming AC power is interrupted. The energy reserve of the DC battery bus then provides power to a number of isolated DC-DC converters (e.g., board mounted power supplies) that convert the DC power from the bus voltage level to a voltage level suitable for the load.
The rectifier generally includes a power factor correction circuit that processes the incoming AC power and produces therefrom high voltage DC power. The rectifier further includes an inverter that generates high frequency AC power from the high voltage DC power, for transmission across an isolation transformer. A rectifier and filter circuit of the rectifier then converts the high frequency AC power into DC power suitable for coupling to the DC battery bus. During a normal mode of operation, the DC power provided to the DC battery bus by the rectifier is used to power the loads and to charge the batteries.
The DC-DC converters generally include an input filter circuit and an inverting circuit that produce high frequency AC power from is the DC power on the DC battery bus for transmission across an isolation transformer. A rectifier and filter circuit of the converter then rectifies the high frequency AC power to produce DC power that powers the load.
Power plants employing the above described DC-bus architecture provide several advantages including relatively high system availability and good decoupling between the various units of load equipment. Inasmuch as the rectifiers and DC-DC converters are coupled to the DC battery bus, however, the architecture requires many components and power conversion stages and is thus inefficient, bulky and expensive.
The aforementioned architectures are directed to delivering DC power to a load. Some telecommunications systems require an AC voltage signal to operate portions of the system equipment, such as ringer circuits or to operate AC powered loads such as air conditioning equipment or computer displays. Another example is a backed-up power system that can be used to power both a wireless base station via a DC output and a hybrid fiber coaxial network via an AC output. In the case of AC powered circuits, the required AC power has very specific voltage magnitude and frequency specifications that are dictated by the equipment. This condition necessitates being able to provide both the required AC and DC power to the system under all conditions including a backup operating condition when the main source of AC input power is not available.
A particular concern is managing the transition from a normal or primary operating mode to a mode requiring the use of a backup power system. A control circuit that is used to manage such a transition is required to detect when there is an absence of a primary source voltage in a primary power system and switch to a backup power system. Typically, the primary power system and the backup power system are voltage sources having a low output impedance. Since the outputs of low output impedance voltage sources cannot generally be directly coupled together without causing serious circulating current problems with probable component damage, the transition must be orchestrated carefully. This requirement typically increases the complexity and therefore the cost of such control systems.
Accordingly, what is needed in the art is a power supply having backup power capability providing an AC output voltage that simplifies the transition between operating modes.